The present invention relates to meltblowing dies used in the meltblowing of thermoplastic materials for applying adhesives, coatings, and nonwoven fabrics. More specifically, the present invention relates to a novel tubular meltblowing die which is economical to manufacture and operate.
Meltblowing is a process for producing nonwoven fabrics by extruding a molten thermoplastic through a row of orifices to form a plurality of molten or semimolten fibers. Heated convergent air streams, referred to as primary air, are directed onto opposite sides of the extruded fibers to attenuate and draw down the fibers to microsized diameters. The attenuation is due to a combination of aerodynamic drag and fiber stretching due to interfiber entanglement. The converging air streams and extruded fibers form a fiber/air stream which is directed onto a rotating collector surface where the fibers deposit in a random way to form a nonwoven fabric or web. The web is held together by a combination of fiber entanglement and fiber cohesive sticking while still in the molten or semimolten state. Meltblown webs are used in a number commercially important applications such as filters, battery separators, petrochemical absorbents, and fabrics to name a few.
Meltblown webs are produced using a meltblowing die which typically comprises a die body, a die tip attached to the body, and converging primary air flow passages flanking the die tip. U.S. Pat. No. 4,986,743 discloses a meltblowing die with an elongate die tip having a triangular or tapered nosepiece which terminates in an apex. The nosepiece has a row of side-by-side orifices drilled along the apex and an internal polymer flow passage which is in fluid communication with the orifices. The internal flow passage registers with a polymer flow passage in the die body so that a pressurized polymer melt flowing from the die body into the die tip is extruded through the orifices to form a plurality of side-by-side fibers. The molten polymer is delivered to the die body by a separate apparatus referred to as an extruder.
U.S. Pat. No. 4,986,743 also teaches the use of die components referred to as air plates or air knives bolted to the die body on flanking sides of the die tip nosepiece. The air plates are elongate plates having a tapered edge and are mounted in relation to the nosepiece so that the tapered edge in combination with the tapered nosepiece form air flow passages which converge onto opposite sides of the die tip orifices. The air flow passages register with flow passages in the die body so that pressurized and heated primary air delivered to the body flows into the air flow passages and is discharged as converging air streams which contact the extruded fibers. The streams attenuate and draw down the fibers and blow the fibers onto a moving collector to form the web. The primary air is usually supplied by a compressor or blower and may be heated before entering the die body using in-line electric or gas heaters.
The extruded molten fibers solidify substantially in the fiber/air stream due to cooler ambient air aspirated into the stream. The prior art also teaches the use of a secondary cooler quenching fluid such as air or water directed onto the fibers as they leave the die. The use of quenching fluid permits higher polymer throughput by providing a higher fiber cooling and solidification rate than possible by the use of aspirated air only.
Meltblowing dies of the type described above are usually constructed from high-quality steel to withstand the elevated temperatures and pressures used in meltblowing. The die components tend to have complex geometric forms requiring extensive and precise machining in their construction. This is particularly true of the die body and the die tip which contain a number of irregular internal flow passages which must properly align and seal when the die is assembled. These factors add significantly to the cost of manufacturing meltblowing dies and, therefore, have an impact on the economics of the process.
In addition to initial manufacturing costs, operating costs associated with the power consumption of the primary air mover can be significant. The primary air flow passages in the die body usually contain a number of irregular bends and obstructions which restrict the air flow. Consequently, a major factor in the power consumption of the primary air mover is the flow energy required to overcome pressure losses encountered in these restricted flow passages. Other operating costs include the power consumed in heating the primary air as well as heating the die body with electric heaters, as is often done. Heating the die body is necessary to maintain the polymer inside the die in the liquid phase. Maintenance costs include the purchase of expensive replacement pans such as the die tip where it is not uncommon for the die tip to become plugged or damaged during operation.
U.S. Pat. No. 4,314,670 discloses an atomizer for atomizing liquid water into a spray of droplets. By atomizing the liquid the resulting droplets may be cooled by the ambient air more rapidly than the liquid as a bulk. The atomizer may be applied to industrial applications which require a fine spray such as cooling liquid-cooled machinery, and producing artificial snow. The atomizer of U.S. Pat. No. 4,314,670 comprises an outer tube having an inner manifold tube disposed longitudinally therein. The outer tube has a faceplate attached to the tube with a tapered slot formed along the faceplate. The manifold has a tapered protrusion having at one end a plurality of relatively large flow openings from the center of the manifold into the protrusion, and at the other end a pair of cantilevered and flexible wall elements attached to the protrusion with screws. The flexible elements protrude into the slot of the faceplate and define therewith an inwardly tapered flow passage on either side of the protrusion. In operation air is delivered to the manifold, flows through the flow openings into the protrusion, flows between the elements and is delivered to the faceplate slot. Liquid water is introduced into the outer tube and flows into the tapered flow passages and converges onto opposite sides of the central air flow. The mixing of the water and air flows causes the water to atomize into droplets. The size of the droplets is controlled by the water pressure in the outer tube which causes the flexible elements to deflect thereby changing the width of the tapered flow passages. The patent further discloses that the liquid and air flow may be interchanged.